1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the speed of a moving body, and more particularly to a method and an apparatus for measuring the speed of a moving body using an accelerometer.
2. Description of the Related Art
As generally known in the art, a navigation system is mounted on a moving body (e.g., a ship, an airplane, an automobile) and used for showing the position of the moving body and providing a route to a destination. The navigation system must be aware of the exact position of the moving body in order to find its position and provide the route to a destination.
Accordingly, the navigation system is usually provided with a positioning device to determine its own position. The positioning devices are classified into two types: a type of devices that determine positioning with aid from outside and another type of devices that determine positioning with an internal sensor. A typical example of the former is a Global Positioning System (GPS) and that of the latter is a Dead Reckoning (DR) system using an inertial sensor.
The DR system, comprising an inertial sensor, uses a gyroscope for sensing the rotational movement of a moving body and an accelerometer for sensing the, linear movement of the moving body to calculate the velocity and position of the moving body. In particular, the velocity of the moving body can be obtained by combining the information on speed from the accelerometer and the information on the direction of movement from the gyroscope.
Conventionally, for an accurate calculation of the velocity of a moving body in a three-dimensional space, three one-axis gyroscopes, as well as three one-axis accelerometers are necessary, installed in orthogonal directions independent from one another. However, the number of sensors may be reduced according to the type of the moving body. For example, in the case of a vehicle, the roll motion, i.e., the rotational movement of the axle (the axle extending between the front and read ends of the vehicle) and the linear movement perpendicular to the ground can be ignored and the number of sensors can be reduced accordingly.
Generally, in order to obtain the travel velocity vector of a vehicle on a road using a DR system with an inertial sensor, the angle of travel direction and the speed in the travel direction must be measured. To measure the angle of travel direction, the DR system requires a gyroscope installed on an axis perpendicular to the plane in which the axle of the vehicle is placed. To measure the speed in the travel direction, the DR system requires not only an accelerometer, installed in the direction of the axle, but also a measured value of the inclination angle of the road.
The reason the DR system has to measure the inclination angle of the road for determining the travel velocity vector of a vehicle is that a gravitational acceleration is included in the measurement from the accelerometer and such a gravitational acceleration can be computed from the inclination angle of the road. The gravitational acceleration is always directed perpendicular to the surface of the earth's ellipsoid (the spherical'surface formed in a direction perpendicular to the gravitational acceleration). As the inclination angle of the road changes, the direction of the axis of the accelerometer (fixed to the vehicle in a direction) mounted on the vehicle also changes. Then, the gravitational acceleration component included in the output from the accelerometer is varied accordingly. As a result, the gravitational acceleration component, which is variable according to the inclination angle of the road, must be removed from the output of the accelerometer, to obtain the pure movement acceleration of the moving body. However, without the information on the inclination angle of the road there can be no distinction between the horizontal movement acceleration component and the vertical gravitational acceleration component in the travel direction and, consequently, the actual speed of the moving body cannot be obtained.
FIG. 1 is a drawing illustrating the concept of gravity compensation for the output from the accelerometer. Referring to the drawing, the inclination angle of the road is defined as the inclination angle relative to the plane perpendicular to the direction ±g of the earth's gravitational acceleration. The angle between the plane 10 perpendicular to the direction ±g of the earth's gravitational acceleration and the plane 20 extending in the travel direction of the vehicle is defined as the inclination angle θ of the road.
As in the case shown in FIG. 1, when a vehicle 30 travels on a plane 20 inclined at an angle θ relative to a plane 10 perpendicular to the direction ±g of the earth's gravitational acceleration, the measurement {right arrow over (a)} from an accelerometer mounted on the vehicle 30 is defined in Equation 1 as follows:{right arrow over (a)}={right arrow over (ra)}+{right arrow over (g)}  (1)
That is to say, the measurement {right arrow over (a)} from the accelerometer mounted on the vehicle 30 includes an actual acceleration {right arrow over (ra)} and the earth's gravitational acceleration component {right arrow over (g)}. The earth's gravitational acceleration component {right arrow over (g)} is measured together with the actual rate of change of the movement speed and, therefore, constitutes a considerable cause of error in measuring the speed.
As a result, the gravitational acceleration {right arrow over (g)} must be subtracted from the measurement {right arrow over (a)} from the accelerometer for an accurate measurement of velocity and the inclination angle of the road must be measured for the gravitational acceleration {right arrow over (g)}. This usually requires the installation of an additional gyroscope or an inclinometer in the DR system.
In the case of a DR system for vehicles, gyroscopes have been used conventionally to measure the inclination angle of the road and, therefore, two or more gyroscopes have been commonly used. In particular, the DR system for vehicles according to the prior art had to comprise a gyroscope for determining the travel direction and another gyroscope for measuring the inclination angle of the road. Basically, the gyroscope is a sensor measuring the rate of change of an angle. Accordingly, if a gyroscope is to be used for measuring the inclination angle of the road in a DR system for vehicles, the output from the gyroscope should be integrated to obtain the inclination angle of the road. As a result, when a gyroscope is used to measure the inclination angle of the road, the error component from the gyroscope tends to be integrated during the integral process. This results in a drawback of an accumulation of errors in estimating the inclination angle of the road over time.
Due to such a drawback, the inclination angle of the road is mainly obtained with the aid of auxiliary sensors, which are free of error-accumulation properties, instead of using gyroscopes only. Additional sensors, such as inclinometers, may be installed to measure the inclination angle of the road accurately. However, in order to minimize the number of sensors, an approach of using an already-equipped accelerometer as the auxiliary sensor is utilized frequently.
FIG. 2 illustrates the procedure for measuring a gravitational component from the output from an accelerometer according to an embodiment of the prior art. In conventional systems, as shown, the output from the accelerometer, comprising both a gravitational acceleration component a and an actual acceleration component b, is made to pass through a Low-Pass Filter (LPF) 40 to measure the gravitational acceleration component having a relatively low frequency, and calculate the inclination angle of the road.
However, although the information on the inclination angle of the road obtained by the above-mentioned method according to the prior art does not suffer from the error-accumulation property, thus obtained information is sensitive to the performance of the accelerometer, while it is insensitive to the change of the inclination angle.
Accordingly, it has been proposed to combine an estimation value from a gyroscope, which is sensitive to an instant change, with that from an accelerometer, which is free of the error-accumulation property, to obtain an estimation value of the inclination angle of the road, which has less error-accumulation and is not insensitive to a change.
In summary, the prior art added gyroscopes to the DR system for vehicles to estimate the inclination angle of the road, because an accurate measurement of the earth's gravitational acceleration, which is needed to compute an accurate inclination angle of the road, is impossible if only accelerometers are used. However, such addition of expensive gyroscopes makes it impossible to realize an inexpensive DR speed measuring device.